Large nonreciprocal absorption and emission of radiation in type-I Weyl semimetals with time reversal symmetry breaking

The equality between the spectral, directional emittance and absorptance of an object under local thermal equilibrium is known as Kirchhoff's law of radiation. The breakdown of Kirchhoff's law of radiation is physically allowed by breaking time reversal symmetry and can open opportunities for nonreciprocal light emitters and absorbers. Large anomalous Hall conductivity and angle recently observed in topological Weyl semimetals, particularly type-I magnetic Weyl semimetals and type-II Weyl semimetals, are expected to create large nonreciprocal electromagnetic wave propagation. In this work, we focus on type-I magnetic Weyl semimetals and show via modeling and simulation that nonreciprocal surface plasmons polaritons can result in pronounced nonreciprocity without an external magnetic field. The modeling in this work begins with a single pair of Weyl nodes, followed by a more realistic model with multiple paired Weyl nodes. Fermi-arc surface states are also taken into account through the surface conductivity. This work points to the promising applicability of topological Weyl semimetals for magneto-optical and energy applications.Comment: 24 pages, 4 figure

Adhesion measurements in MOS2 dry lubricated contacts to inform predictive tribological numerical models : comparison between laboratory-tested samples and ball bearings from the niriss mechanism

International audiencePredicting the tribological behaviour of dry lubricants remains difficult because it greatly depends on their mechanical and physicochemical environment. While it is difficult to analytically model dry lubrication, Discrete Element Method (DEM)-based modelling has been able to provide valuable insight into the tribological behaviour of dry lubricated contacts. The present study aims to experimentally define interactions between the discrete elements used for simulating different materials in contact, in order to accurately model and predict the tribological behaviour of dry lubricants. Those interactions are here defined by using the work of adhesion (W) between engineering materials: AISI440C, pristine MoS2 coating, as well as the related transfer film. A method was developed and applied on regular laboratory tribological test samples and ball bearings from the Near Infrared Imager and Slitless Spectrograph (NIRISS) instrument of the James Webb Space Telescope. Measured W values were consistent between all worn surfaces. The first DEM modelling results exhibit behaviours similar to those observed experimentally including surface plasticization and transfer

Nonreciprocal and Exotic Radiative Transfer in Type-I Magnetic Weyl Semimetals

The classical theory of radiative heat transfer has proven extraordinarily useful, both in advancing fundamental physics through the discovery of quantum mechanics and in developing practical applications. However, the classical theory can break down, opening up new opportunities for energy production and management. Kirchhoff’s law of radiation breaks down in systems with broken Lorentz reciprocity, which are characterized by asymmetric dielectric tensors and broken time reversal symmetry. This is typically achieved using an external magnetic field, but a recently discovered family of quantum materials—Weyl semimetals—can possess intrinsic nonreciprocity due to the Berry curvature, an internal, pseudo-magnetic field. In this dissertation, we explore thermal radiation from type-I magnetic Weyl semimetals in planar configurations in the far- and near-fields. First, we demonstrate that a planar interface between air and a Weyl semimetal can intrinsically violate Kirchhoff’s law of radiation, and that nonreciprocal surface plasmon polaritons (SPPs) excited on this interface drive the nonreciprocity in the radiative heat transfer. In addition, we propose a physical mechanism for the nonreciprocity in terms of the forces felt by electrons as a result of the Berry curvature. Further leveraging the nonreciprocal SPPs, we explore the radiative heat transfer between two planar Weyl semimetal surfaces in the near-field (where Planck’s law also breaks down). To accurately compute the nearfield radiative heat transfer, we include the effects of Fermi arc surface states, a hallmark of Weyl semimetals. We show that—in contrast to far-field radiative heat transfer—Fermi arc surface states play a significant role in the near-field and that the heat flux between the two Weyl semimetals can be tuned via twisting their surfaces in the lateral direction. This twist-controlled heat flux raises questions about the role of the configurational symmetry of the two-body system, and we argue that using materials with asymmetric dielectric tensors is necessary but not sufficient to realize nonreciprocal radiative heat transfer. The system must also possess some broken configurational symmetry, namely inversion symmetry. Finally, we identify opportunities for new discoveries in thermal radiation from materials with broken inversion and time reversal symmetry.S.M

Intrinsic nonreciprocal reflection and violation of Kirchhoff's law of radiation in planar type-I magnetic Weyl semimetal surfaces

This work demonstrates that Kirchhoff's law of radiation, stating that the spectral directional emissivity and absorptivity of a surface are equal at thermal equilibrium, can be violated in planar surfaces without an external magnetic field or structures such as gratings. Modeling a type-I magnetic Weyl semimetal with an antisymmetric dielectric tensor, we show an intrinsic violation of Kirchhoff's law due to nonreciprocal surface polaritons induced by the Berry curvature and anomalous Hall velocity. This work provides a simple way to physically realize the violation of Kirchhoff's law

High temperature microtribological studies of MoS2 lubrication for low earth orbit

International audienceMolybdenum disulfide is one of the most common lubricant coatings for spaces ystems but it displays enormous susceptibility to environmental conditions making it hard to predict performance throughout the entire lifetime. The majority of mechanisms for space operate in low Earth orbit where temperatures typically reach 120 ◦C along with exposure to highly reactive atomic oxygen which can be detrimental to lubricant performance. In the present study, a MoS2 lubricant coating is tested using friction force microscopy under different environmental conditions including air and dry nitrogen environments with temperatures ranging from 25 ◦C to 120 ◦C. The increased temperaturewas found to be beneficial for friction behaviour inair up to 100◦C as ambient humidity is removed from the contact, but higher temperatures become detrimental as increased reactivity leads to oxidation. These competing effects resulted in a minimum coefficient of friction at 110◦C in the air environment. The high temperature also increases the wear of the coatings as the intrinsic shear strength decreases with thermal energy which in turn disrupts tribofilm formation leading to increased friction. The run-in duration and magnitude are both found to decrease with temperature as the energy barrier to optimal reconfiguration is reduced. Finally, contextualization of the present findings for mechanisms operating in low earth orbit is discussed

Work of Adhesion Measurements of MoS 2 Dry Lubricated 440C Stainless Steel Tribological Contacts

International audienceThe tribological behavior of dry lubricants depends on their mechanical and physicochemical environment, making it difficult to predict in practice. Discrete Element Method-based modeling has been one successful approach to provide valuable insight into the tribology of dry lubricated contacts. However, it requires well-defined interactions between discrete elements, in particular between those simulating different materials. Measuring the properties governing those interactions, such as the work of adhesion (W), is therefore critical. The present work describes a method for measuring the W between AISI440C steel and MoS 2-based coatings used in spacecraft. Using Atomic Force Microscopy local asperity and adhesion measurements, the W between steel microbeads and MoS 2 coatings is determined at different stages in its wear life. The distributions of W values in the worn coatings and pristine coatings agree well with earlier Time-of-Flight Secondary Ion Mass Spectroscopy studies on the physicochemistry of the samples, as well as contact angle measurements. Additional measurements between the same materials on a ball bearing from a real life-test unit of a spacecraft instrument also show a similar W distribution, suggesting that the approach used here provides relevant data for use in numerical simulations

Bioinspired kirigami metasurfaces as assistive shoe grips

© 2020, The Author(s), under exclusive licence to Springer Nature Limited. Falls and subsequent complications are major contributors to morbidity and mortality, especially in older adults. Here, by taking inspiration from claws and scales found in nature, we show that buckling kirigami structures applied to footwear outsoles generate higher friction forces in the forefoot and transversally to the direction of movement. We identified optimal kirigami designs capable of modulating friction for a range of surfaces, including ice, by evaluating the performance of the dynamic kirigami outsoles through numerical simulations and in vitro friction testing, as well as via human-gait force-plate measurements. We anticipate that lightweight kirigami metasurfaces applied to footwear outsoles could help mitigate the risk of slips and falls in a range of environments

Light-degradable hydrogels as dynamic triggers for gastrointestinal applications

Copyright © 2020 The Authors Triggerable materials capable of being degraded by selective stimuli stand to transform our capacity to precisely control biomedical device activity and performance while reducing the need for invasive interventions. Here, we describe the development of a modular and tunable light-triggerable hydrogel system capable of interfacing with implantable devices. We apply these materials to two applications in the gastrointestinal (GI) tract: a bariatric balloon and an esophageal stent. We demonstrate biocompatibility and on-demand triggering of the material in vitro, ex vivo, and in vivo. Moreover, we characterize performance of the system in a porcine large animal model with an accompanying ingestible LED. Light-triggerable hydrogels have the potential to be applied broadly throughout the GI tract and other anatomic areas. By demonstrating the first use of light-degradable hydrogels in vivo, we provide biomedical engineers and clinicians with a previously unavailable, safe, dynamically deliverable, and precise tool to design dynamically actuated implantable devices

Temperature-responsive biometamaterials for gastrointestinal applications

We hypothesized that ingested warm fluids could act as triggers for biomedical devices. We investigated heat dissipation throughout the upper gastrointestinal (GI) tract by administering warm (55°C) water to pigs and identified two zones in which thermal actuation could be applied: esophageal (actuation through warm water ingestion) and extra-esophageal (protected from ingestion of warm liquids and actuatable by endoscopically administered warm fluids). Inspired by a blooming flower, we developed a capsule-sized esophageal system that deploys using elastomeric elements and then recovers its original shape in response to thermal triggering of shape-memory nitinol springs by ingestion of warm water. Degradable millineedles incorporated into the system could deliver model molecules to the esophagus. For the extra-esophageal compartment, we developed a highly flexible macrostructure (mechanical metamaterial) that deforms into a cylindrical shape to safely pass through the esophagus and deploys into a fenestrated spherical shape in the stomach, capable of residing safely in the gastric cavity for weeks. The macrostructure uses thermoresponsive elements that dissociate when triggered with the endoscopic application of warm (55°C) water, allowing safe passage of the components through the GI tract. Our gastric-resident platform acts as a gram-level long-lasting drug delivery dosage form, releasing small-molecule drugs for 2 weeks. We anticipate that temperature-triggered systems could usher the development of the next generation of stents, drug delivery, and sensing systems housed in the GI tract.Bill and Melinda Gates Foundation (Grants OPP1139921 and OPP1139937)NIH (Grant EB000244